This invention relates generally to the preservation of agricultural products. More particularly, the invention relates to compositions for surface coating for making possible the preservation of agricultural products over long periods, to a process for preserving agricultural products through the use of these compositions, and to agricultural products thus processed for preservation.
Among agricultural products, vegetables, fruits, and cereals or grains, even during their storage after harvesting, carry on living functions, inspire oxygen from the outside air to carry out respiratory action, and continue vital functions and phenomena as they consume nutrients stored within their structures.
Vegetables, fruits, and cereals (hereinafter referred to as vegetables and (or) the like) after harvesting may be stored in fresh state for long periods preserving them in a state wherein they are sustained at minimal level of vital functions and are rendered into a dormant state, the consumption of stored nutriments in their structure is reduced to a minimum degree, and their structural compositions immediately after harvesting are maintained as much as possible.
In general, long-period preservation can be accomplished by utilizing such principles as (1) suppression of respiration, (2) prevention of evaporation of water content, and (3) prevention of change of quality or deterioration due to microorganisms.
Examples in the prior arts of utilization of these principles in specific form for practical preservation are (1) suppression of respiration by low-temperature storage, (2) suppression of respiration by lowering the partial pressure of oxygen in the preserving atmosphere, (3) suppression of respiration and prevention of evaporation of water content by packaging in film envelopes, and (4) suppression of respiration and prevention of evaporation of water content by coating the surfaces of the commodities.
In the method (1) of suppressing respiration by low-temperature storage, the commodity, or vegetable and the like, is preserved at such a low temperature as possible after harvesting without physiological damage due to low temperature with the aim of preserving the commodity over a long period. In general, the respiratory activity of a plant becomes very low at temperatures close to 0.degree. C, and the quantity of respiration increases 2 to 4 times with a temperature rise of 10.degree. C; therefore, low-temperature storage is a very effective measure for long period preservation. This method, however, requires great expense for installation of equipment, and, moreover, the maintenance of low-temperature conditions also adds to the operational cost. Furthermore, when, at the stage of transferring the commodity from cold storage to the consumer, the maintenance of cooling is interrupted and the temperature of the commodity is returned to temperature of outside air, the surfaces of the vegetables and the like get wet due to dew collecting thereon, whereby the product quality deteriorates rapidly.
In the method (2) of suppressing respiration by lowering the partial pressure of oxygen in the atmosphere surrounding the commodity, the property of plants such as vegetables whereby their quantity of respiration varies with the partial pressure of oxygen in the surrounding atmosphere, becomes progressively small as the partial pressure of oxygen in the air becomes lower than 0.2 atmospheres, and decreases rapidly when this partial pressure becomes less than 0.05 to 0.1 atmospheres. For example, the so-called CA (controlled atmosphere) storage method, wherein a few percent to 20 percent of carbon dioxide gas is admixed into the air in the storage chamber of warehouse to lower the partial pressure of oxygen with the aim of long-period preservation, is regarded as a very good storage method. However, similarly as in the above described low-temperature storage method, this CA storage method entails high cost for equipment and operation, and, furthermore, the beneficial effect of the preservation is lost by the return to the ordinary atmosphere in the distribution process subsequent to shipment from the storage warehouse.
In the method (3) of suppressing respiration and preventing moisture evaporation by packaging in a film envelope, vegetables and the like are stored in envelopes made of thin films of resins such as polyvinyl chloride, polyethylene, and polypropylene. This method is initially effective in preservation in that, within the small, limited system in each envelope, the partial pressure of oxygen is lowered by the carbon dioxide gas exhaled by the contents of the envelope. However, the carbon dioxide gas thus discharged progressively accumulates in the envelope to cause so-called accumulative injury or damage, and, at a certain time, a sudden lowering of the product quality or decaying occurs. Of course, selection of the thickness and material of the envelope film or the supplementary measure of aiding the flow of air by mechanically forming small holes therein has been resorted to overcome the disadvantage, but as the degree of ripening of the vegetable or the like contained in the envelope progresses, fluctuations in the required oxygen quantity, the discharged carbon dioxide gas quantity, and, further, the quantity of moisture evaporated off occur, and it is extremely difficult to exercise fine control simultaneously over all of these conditions.
The method (4) of suppressing respiration and preventing moisture evaporation by surface coating has the following features. Ordinarily, a higher plant grown on land inhales oxygen necessary for cellular respiration from the outside and discharges carbon dioxide gas to the outside, the flowing in and discharging of gases necessary for respiration being accomplished through the epidermal cells having stigmas and lenticels. Therefore, by decreasing the cross-sectional areas of these stigmas and lenticels, the quantity of oxygen inhaled into the plant structure can be restricted without changing the partial pressure of the oxygen in the outside air, and, as a result, the quantity of respiration is reduced. Accordingly, as methods of utilizing this phenomenon for preservation, a large number of processes for coating films by some measure on the surfaces of vegetables and the like have been proposed. Examples of these proposals are those set forth in the specifications of Japanese Pat. Publication Nos. 32344/1973, 946/1973, 18467/1973, 5030/1973, and 10683/1960 and U.S. Pat. Nos. 2,700,025, 2,755,189, 2,872,325, 3,410,696, 2,961,322, and 2,346,755.
These processes proposed heretofore, generally comprise forming a film of hydrophobic wax or resin on the surface of the vegetables or like in order to prevent lowering of the degree of freshness thereof due to evaporation of water content or comprise forming a film of any of various synthetic resins on the surfaces of the vegetables or the like for the purpose of controllably suppressing passage of gases, by utilizing the differences in gas permeability due to the molecular structures of the synthetic resins.
However, for accomplishing fine control of physiological functions in accordance with each and every kind of vegetable or the like by these proposed processes, ample preservation effect would not be attained with films of uniformly standardized materials. For example, in the case where a film of very low gas permeability is formed to coat a vegetable or the like of relatively intense respiration action, and the minimum quantity of oxygen required for respiration cannot be inhaled in, the vegetable or the like undergoes anaerobic respiration, that is, respiration between molecules, and as a metabolic product, alcohol is produced instead of carbon dioxide gas and water. As a result, an abnormal odor impairing the taste of the vegetable or the like as food is generated, and the object of preservation would not be achieved. On the other hand, in the opposite case wherein a film of relatively high gas permeability is formed as a film to coat a vegetable or the like of small respiration quantity, gas necessary for respiration freely flows in, and the effect of preservation due to the film is not exhibited.
On one hand, an egg, which is herein regarded as one of agricultural products, comprises a blastodisc or embryo, egg white or albumen, egg yolk and an eggshell. The eggshell has numerous micropores, through which moisture and carbon dioxide gas are discharged to the outside during storage, and, at the same time, microorganisms infect into the egg from the outside through these micropores. The evaporation of moisture from an egg is principally the evaporation of the water in the albumen to the outside, and this gives rise to a decrease in the weight of the egg, an expansion of the air shell, and a lowering of the product quality. Furthermore, when the carbon dioxide gas dissolved in the albumen penetrates to the outside, the pH rises, and it becomes impossible to prevent the propagation in the egg of the microorganisms which have infected thereinto from the outside. Consequently, the preservability of the egg decreases remarkably.
The principles of long-period preservation of eggs of fowl such as chickens and quails, which eggs after being laid are used especially for food, are substantially the same as those in the case of vegetables and the like, the important measures based on these principles being (1) suppression of exhalation of carbon dioxide gas to the outside, (2) prevention of evaporation of moisture and (3) prevention of infection and propagation of microorganisms.
Examples of preservation methods based on these principles and heretofore proposed are (1) prevention of infection and propagation of microorganisms by low-temperature storage and (2) suppression of exhalation of carbon dioxide gas to the outside, prevention of evaporation of moisture, and prevention of infection of microorganisms by surface coating.
The former method (1) comprises conveying laid eggs to an egg storage chamber at low temperature, carrying out processes such as inspecting, washing, and classification of the eggs, packaging the eggs in boxes, transporting the eggs thus packaged to consumption areas by refrigerated trucks, vans or lorries, and carrying the eggs to refrigerated show cases of retail stores. In an imperfect distribution system, as those existing at present, however, water vapor in the atmosphere condensed on the eggshells when the eggs contact the outside air during distribution, and, as a consequence, the eggs assume a state resembling perspiration or sweating. The net result, contrary to expectation, is that the preservability is lowered. Furthermore, this method tends to be excessively expensive and is difficult to implement under the present circumstances.
The latter method (2) comprises coating the surfaces of eggs with a film to preserve the eggs by utilizing the properties of the film. For this purpose, a number of processes have been proposed, examples of which are those disclosed in Japanese Pat. Publication Nos. 5027/1957, 6226/1969, and 20150/1968. These known processes are characterized by the forming on the egg surface of a film of a hydrophobic material such as wax or paraffin for the purpose of preventing lowering of the freshness of the egg due to evaporation of water or by the forming on the egg surface of a film of any of various synthetic resins for the purpose of controllably suppressing the passage of gases, the differences in gas permeabilities due to the molecular structures of these synthetic resins being utilized. By these processes, however, control of the discharge of the carbon dioxide gas of the egg to the outside, the evaporation of water, and prevention of infection and propagation of microorganisms would not be accomplished simultaneously and efficiently. This problem is substantially the same as that described with respect to vegetables and the like.